In a recent paper in the Journal of Vacuum Science and Technology A, the researchers describe how they developed, deposited and tested their "Hi Low Hi Low" multilayer broadband anti-reflection coating for cadmium telluride (CdTe) PV cells that significantly reduces reflection, works effectively over the entire range of light wavelengths needed, and negates the previously seen energy loss with a comparable increase in device efficiency.
Most solar cells in use today do not have any means of controlling reflection. A few modules have their glass surface coated with a single layer of anti-reflection material that only efficiency eliminates reflection at a single wavelength of light. Additionally, this type of coating is porous and can become filled with water when exposed to rain and snow, degrading its anti-reflective properties and weakening its durability.
"We decided that what was needed was a multilayer coating to achieve broadband anti-reflection; in other words, use the light interference from each layer to minimize the overall reflectivity," said Michael Walls, professor of photovoltaics at Loughborough and corresponding author on the JVSTA paper.
Walls and his colleagues used a deposition method called magnetron sputtering, an industrial coating technique used by glass manufacturers, to apply four layers of material-two with a high refractive index (zirconia) and two with a low refractive index (silica)- to the CdTe solar cells. The alternating layers of high and low refractivity gives the design its understandable nickname, "Hi Low Hi Low."
Walls reported that the multilayer coating - only 250 nanometers thick (30 times smaller than a red blood cell) - possesses some extremely positive properties.
"Reflection is reduced by about 70 percent and works for the entire range of light wavelengths [350 to 850 nanometers] used by CdTe cells, and the module efficiency increases by 3.6 percent, a significant improvement over untreated solar cells," he said. "Additionally, the anti-reflective coating is non-porous, remains stable in all weather conditions, resists scratching and can withstand temperatures greater than 550 degrees Celsius."Walls said that the Loughborough research team will next try to produce other broadband anti-reflection coating designs that will improve the efficiency of all types of PV solar technologies, not just systems using CdTe cells. Read More